Method of making a stepper motor

ABSTRACT

A multi-phase stepper motor having an annular permanent magnetic rotor and a plurality of matingly engageable stator phase assemblies around the rotor. Each of the phase assemblies comprises a pair of annular pole pieces including interleaved salient stator poles. The stator pole pieces in each pair are mated together in an opposed relationship to form an annular space between them to receive an energizing winding, and the winding for each pair is of a different phase. Depending upon the number of phase assemblies, the machine can operate in more than two phases. The diameter of the rotor is between about 55 percent and about 75 percent of the motor diameter, and approximately 40 percent more torque is produced than with equivalent sized conventional motors.

This application is a division of application Ser. No. 121,170, filedDec. 30, 1987, now U.S. Pat. No. 4,841,189.

BACKGROUND OF THE INVENTION

This invention relates to electrical rotating machines and moreparticularly to electric motors of the type in which the rotor of themotor turns in discreet increments or steps, and to a method for makingsuch motors. The invention more specifically relates to a stepper motorutilizing a rotor preferably having a diameter that is at least 60% ofthe motor diameter to provide substantially more torque thanequivalent-sized conventional motors.

The use of multiple field coils with a single non-salient pole rotor ina stepper motor is well-known in the art and is shown by such priorpatents as U.S. Pat. No. 4,241,270 and U.S. Pat. No. 4,274,026. Toprevent radial displacement of the stator within the motor assembly,stator pole pieces have heretofore been notched such that adjacentstator pole pieces may be frictionally interlocked. Representativemotors of this type are disclosed, for example, in U.S. Pat. No.4,333,026 and U.S. Pat. No. 3,495,113. These prior art references,however, do not provide for any number of individual stator phaseassemblies including a plurality of stator pole pieces and a notchedfield ring to be interlocked together. Thus, it would be advantageous ifone could link more than two stator assemblies, with each assembly beingof different phase such that the motor could operate in more than twophases. In addition, it is desirable to provide a stepper motor assemblywherein the same parts can be used to achieve more than two phases, andhence, reduce tooling costs.

The stepper motor of the present invention also includes, in a preferredembodiment, a rotor having a diameter that is at least approximately 55%of the motor diameter and thus produces substantially more torque thanequivalent sized conventional motors.

The present invention is directed towards solving these problems andprovides a workable and economical solution to them.

OBJECTS OF THE INVENTION

It is therefore a general object of the present invention to provide animproved stepper motor and method of making the same.

It also an object of the present invention to provide a stepper motorwhich is capable of linking at least two stator pole pieces of differentphases together such that the stepper motor is capable of operating ingreater than two phases.

It is a further object of the present invention to provide a steppermotor which is made up of a plurality of stator phase assemblies eachincluding two pole pieces forming an opening for a coil of wire andfurther including a notched and slotted field ring with each of thestator phase assemblies being interlocked relative to one another.

It is still a further object of the present invention to provide astepper motor with identical individual phase assemblies to therebyreduce tooling costs and facilitate manufacture.

It is yet a further object of the present invention to provide a steppermotor having notches placed on each stator pole assembly to give properphase relationships between the pole plates within an assembly andbetween adjacent assemblies.

It is a still further object of the present invention to provide astepper motor which has a rotor diameter which is at least approximately55% of the motor diameter and thus produces substantially more torquethan equivalent sized conventional motors.

SUMMARY OF THE INVENTION

The foregoing and other objects are accomplished by means of theelectric rotating machine and method in accordance with the presentinvention. In a preferred embodiment, the machine includes an annularpermanent magnetic rotor having rotor poles of alternating polarityaround its circumference. One or more outer stator phase assemblies areprovided which each include a pair of stator pole pieces of annularconfiguration. Each stator pole piece has spaced-apart axially extendingstator poles along its outer periphery in magnetic flux relationshipwith the rotor. The stator poles of one of the pair of pole pieces areof an opposite polarity and are interleaved with the stator poles of theother respective pole piece. Annular energizing means are providedsurrounding the entire periphery of the rotor and are disposedsubstantially entirely within the annular space between the pair ofstator pole pieces and an annular field ring for producing a magneticfield in the salient stator poles of the pair of pole pieces. Theannular field ring has a plurality of notches and projections extendingfrom the side edges of its outer peripheral surface. These notches andprojections permit identical outer stator phase assemblies of differentphases to be frictionally interlocked thereby providing a self-containedmotor assembly which can operate in more than two phases.

In several advantageous arrangements in accordance with the invention,the annular field ring additionally includes a series of peripheralslots in the center of the ring which each accommodate two projectingtabs on the respective pole pieces. The field ring is fabricated from aflat blank which is rolled around the assembled pole pieces to positionthe tabs within the slots and thereby further facilitate the manufactureof the motor with the pole pieces in precise alignment.

The adjacent outer stator phase assemblies of the motor are frictionallyinterlocked at a mechanical displacement of one-half the stator toothpitch (90 electrical degrees for a two-phase motor). This displacementserves to shift the direction of the stator flux passing through therotor-stator air gap as the flux builds up during starting. Accordingly,this shift in direction due to the delayed start-up of the stator fluxof the stator poles will impart a unidirectional starting characteristicto the motor in a manner well understood by those conversant in the art.

The electrical rotating machine of some embodiments of the presentinvention also includes a first and a second mounting plate. Both ofthese mounting plates are provided with a plurality of slots which maybe securely retained by projections extending from the outer statorphase assemblies to thereby self-contain the rotor and stator assemblieswithin the electric rotating machine. One of the mounting plates isprovided with a plurality of screw holes such that it may be mounted andsecured at a desired location.

Furthermore, the electric rotating machine of several good embodimentsof the invention includes a rotor diameter that is at leastapproximately 55% of the motor diameter, thus producing about 40% moretorque than equivalent-sized conventional motors.

The above and other objects, features and advantages of the presentinvention will be apparent from the following detailed description ofillustrative embodiments thereof, which is to be read in conjunctionwith the accompanying drawings, wherein:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of the stepper motor inaccordance with an illustrative embodiment of the present invention.

FIG. 2 is a rear perspective view of the stepper motor shown in FIG. 1.

FIG. 3 is a sectional view of the stepper motor taken along line 3--3 ofFIG. 2.

FIG. 4 is a sectional view of the stepper motor taken along line 4--4 ofFIG. 2.

FIG. 5 is a chart plotting rotor diameter versus output torque of thestepper motor.

FIG. 6 is a fragmentary perspective view showing a method for assemblingcertain components of another illustrative embodiment of the invention.

DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS

Referring now to the drawings in detail, and initially to FIGS. 1through 4, an electrical motor 10 constructed in accordance with apreferred embodiment of the present invention is illustrated. Thepresent invention, while of general application, is particularly wellsuited for use as a stepper motor, i.e., an electrical rotating machinein which the rotor of the machine runs in discrete increments or steps.

In this preferred embodiment, the motor 10 includes a permanent magnetrotor 12 having a set of non-salient permanent magnet rotor poles ofalternating polarity around its outer periphery. Non-salient rotor polesare preferred because of the decreased cost of such rotors over salientrotor poles or individual permanent magnets supported in an annularnon-magnetic carrier. The permanent magnet rotor 12 has 12 non-salientnorth poles and 12 non-salient south poles alternating around its outerperiphery, for a total of 24 poles. The total number of rotor poles can,of course, be any even number.

The rotor 12 is supported by a wheel 14 fixedly mounted to the interiorof rotor 12 (see FIG. 3). The wheel 14 has an opening 15 through whichthe rotor shaft 16 is centrally mounted within the stepper motor. A pairof bearing bosses 18a and 18b are positioned on opposite side surfaces19a and 19b, respectively, of the wheel 14. These bearing bosses 18a and18b include annular openings 20a and 20b through which the rotor shaft16 is inserted, and hence, the openings provide a retention member forretaining the rotor shaft 16 in its centralized position within themotor.

The motor 10 has at least one outer stator phase assembly 22 of annularconfiguration disposed generally concentrically around the outerperiphery of rotor 12. In the embodiment shown in FIGS. 1 through 4, twoof these outer stator phase assemblies 22 and 22' are shown interlockedrelative to one another. As will be discussed herein, the interlockingcapabilities of the outer stator phase assemblies of different phasespermits the interconnection of two or more outer stator phase assembliesand thus provides for operation of the motor in two or more phases.Since the two outer stator phase assemblies shown in the drawings areidentical and interchangeable, the drawings illustrate the correspondingcharacter references for the second outer stator phase assembly with a"'" symbol. However, the character references, such as 22 and 22°, willbe collectively referred to as 22, unless otherwise disclosed herein.

Each of the outer stator phase assemblies 22 includes a pair ofcup-shaped stator pole pieces 24a and 24b. The pole pieces haverespective annular bases 25a and 25b. Each of the stator pole pieces haspreferably 12 integrally formed stator teeth, although any number ofstator teeth may be utilized, and the teeth extend at right angles fromthe inner periphery of their corresponding bases. The stator pole pieces24a and 24b are identical, thus achieving economies in manufacturing andspare parts inventory.

The teeth 26a of stator pole piece 24a extend downwardly from base 25a,as viewed in FIG. 1, with the space between teeth 26a forming aninverted V-shaped notch 27a. Teeth 26b extend upwardly from base 25b ofpole piece 24b, the space between adjacent teeth 26b forming a V-shapednotch 27b. In assemblage, the cup-shaped pole pieces 24a and 24b aremated together in opposed relationship with the teeth 26a insertedwithin the V-shaped notches 27b of rotor pole piece 24b and the teeth26b of rotor pole piece 24b inserted within the V-shaped notches 27a ofrotor pole piece 24a.

In this configuration, the pole pieces 24a and 24b form an annular spacebetween them to receive energizing means comprising an annular woundcoil of wire 30. This winding 30 is disposed substantially entirelywithin the annular space, as is shown in cross-section in FIGS. 3 and 4.The winding 30 is supplied with single phase current pulses by leadwires 32, thus magnetizing the 12 poles of pole piece 24a with onepolarity and the 12 poles of pole piece 24b with the other polarity.

In order to reliably support the stator phase assemblies 22 within thestepper motor 10, each phase assembly 22 has a field ring 34 whichprovides an outer backing for the assembly. In order to link more thanone stator phase assembly 22 together, such that the motor 10 canoperate in two, three, four, five, etc. phases, each field ring 34includes a plurality of projections 36 and notched recesses 38 along thefirst and second peripheral side edges 40a and 40b of the ring. Uponproper orientation, the projections 36 on the side surface 40a of onefield ring 34 will interlock within the recesses 38 in the second sidesurface 40b of another field ring. Similarly, upon proper orientation,the projections 36 on the side surface 40b of one field ring willinterlock within the recesses 38 in the side surface 40a of anotherfield ring. In this configuration, multiple field rings may be stackedand frictionally interlocked within the motor assembly such that eachstator phase assembly 22 can be of a different phase and more than twophases can be attained within each motor assembly.

Each of the stator pole pieces 24a, 24a', 24b and 24b' is provided withfour outwardly extending tabs 39a, 39a', 39b and 39b', respectively.These tabs are coplanar with the corresponding bases of the pole pieces,and the tabs on each pole piece are positioned at ninety mechanicaldegree intervals. As best seen in FIG. 2, during the assembly of themotor the tabs 39a' and 39b on the inner pole pieces 24a' and 24b(FIG. 1) are oriented in overlapping relationship with each other andare inserted in the openings formed by the inwardly facing recesses 38and 38' in the field rings 34 and 34'. The tabs 39a and 39b' on theouter pole pieces 24a and 24b' similarly are disposed within theoutwardly facing recesses in the field rings.

As a result of this interlocking feature of the field rings 34, eachstator phase assembly 22 will have its stator pole pieces at a 90electrical degree index spacing from the stator pole pieces of anadjacent stator phase assembly. As a direct result thereof, thisdisplacement will impart a unidirectional characteristic to the motor ina manner well understood by those conversant with the art. In addition,the displacement of the stator pole pieces creates a self-startingcharacteristic.

In order to securely mount the stator phase assemblies 22 and the rotorassembly 12 within the stepper motor, there are provided front and rearmounting plates 44 and 46, respectively. The rear mounting plate 46includes a disc-shaped base 48 and a bearing boss 50 extending inwardlyfrom the base 48. The bearing boss 50 includes an aperture 52 (FIG. 2)through which the rotor shaft 16 is centrally mounted. The rear mountingplate also includes a plurality of grooves 54 around its periphery whichreceive the projections 36 on one of the field rings 34 such that theplate is securely retained within the motor assembly.

In the illustrated embodiment the front mounting plate 44 is of agenerally diamond or pear shape, although in other embodiments the platemay be of substantially any configuration depending upon therequirements of the user. The plate 44 includes a base 56 and a bearingboss 58 extending from the base. The bearing boss 58 is provided with anaperture 60 through which the rotor shaft 16 is centrally mounted. Aplurality of grooves 62 similar to the grooves 54 are provided in theperiphery of the plate 44 which receive the projections 36 on the sideedges 40a and 40b of the field ring 34. In this manner, the plate 46 maybe reliably mounted adjacent the stator phase assembly 22' within themotor assembly. As a result of this configuration of the mountingplates, rotor, and stator phase assemblies, the motor assembly isentirely self-contained.

The motor 10 has about 40 percent more torque than any knownequivalent-sized motor. Since the maximum torque for a given size motoris a function of the rotor diameter versus the volume of steel versusthe volume of copper, if there is too much copper or too much steel, theoutput torque decreases. Conversely, if the rotor diameter can beincreased in a given motor assembly while not unduly affecting theamount of copper and steel, the torque will correspondingly increase.

FIG. 5 is a chart showing the relationship between rotor diameter andoutput torque for a motor having a diameter of 1.4 inches and a fixedpower input. The curve 65 in FIG. 5 demonstrates that the output torquepeaks when the diameter of the rotor is about 0.90 inches. Thiscorresponds to a rotor diameter which is approximately 64 percent of thediameter of the motor. Although the torque begins to drop off when thediameter of the rotor exceeds about 0.950 inches or approximately 68percent of the motor diameter, the torque remains at a comparativelyhigh level until the rotor diameter reaches 1.05 inches or approximately75 percent of the diameter of the motor. For smaller rotors the torquealso is comparatively high until the rotor diameter drops below 0.75inches or approximately 55 percent of the motor diameter. By maintainingthe rotor to motor diameter ratio within the range of from about 55percent to about 75 percent, particularly good results are achieved, andthe output torque is approximately 40 percent higher than conventionalequivalent-sized motors which customarily have a rotor diameter that isabout 40 to 50 percent or less of the diameter of the motor.

The rotor 12 may be one unitary structure cooperating with all of thestator phase assemblies, as is shown in FIG. 1, or each individualstator phase assembly may be provided with a separate rotor as isindicated by the broken line 13 through the rotor 12 in FIG. 1.

Similarly, the motor may be provided with a separate field ring 34 or34' for each of the stator phase assemblies, or a single field ring mayenclose all of the phase assemblies. Referring to FIG. 6, for example,there is shown a single field ring 70 that cooperates with each of thestator assemblies 22 and 22'. The field ring 70 comprises a flat stripof cold rolled steel or other magnetic material that is provided withlongitudinally extending slots 71 spaced along the center portion of thering. Each of these slots has a width that is twice the thickness of thetabs 39 and 39' on the pole pieces 24a' and 24b of the respective phaseassemblies. Projections 76 along the longitudinal edges of the fieldring 70 interleave with the corresponding grooves 54 and 62 (FIG. 2) onthe mounting plates 46 and 44 or on adjacent field rings depending uponthe number of phase assemblies in the motor.

To assemble the pole pieces 24a' and 24b, the pole pieces are fed bygravity down chutes 80 and 81, respectively, to a pair of opposedmandrels 83 and 84. The mandrels 83 and 84 are then moved axially towardone another to position the pole pieces 24a' and 24b in back-to-backcontact with each other within a shaping die 85. The pole pieces arewelded together within the die 85 with the tabs 39 and 39' incoextensive overlapping relationship with each other.

The mandrels 83 and 84 then rotate the assembled pole pieces 24a' and24b while the field ring strip 70 is introduced into the die 85. As thestrip 70 moves into the die 85, it is rolled around the pole pieces 24a'and 24b such that the strip is given a cylindrical configuration witheach pair of the aligned tabs 39 and 39' disposed in one of the slots 71in the strip. The tabs 39 and 39' are located on the pole pieces inposition to automatically provide the required phase displacementbetween the assemblies 22 and 22'. The arrangement is such that the costof manufacturing the motor is substantially reduced while at the sametime providing a precise and reliable method of aligning the phaseassemblies.

In the embodiment of FIGS. 1-4 the field rings 34 and 34' and themounting plates 44 and 46 serve as a multi-part housing for the machine.Similarly, in the embodiment of FIG. 6 the multi-part housing is formedby the field ring 70 in cooperation with the mounting plates. In each ofthese embodiments the component parts of the housing are locked togetherby the various protrusions, tabs and grooves to reliably retain theparts in position.

The terms and expressions which have been employed are used as terms ofdescription and not of limitation, and there is no intention in the useof such terms or expressions of excluding any equivalents of thefeatures shown and described or portions thereof. Although illustrativeembodiments of the invention have been described with reference to theaccompanying drawings, it is to be understood that various changes andmodifications can be made therein without departing from the scope orspirit of the invention.

What is claimed is:
 1. In a method of making an electric rotatingmachine having an annular permanent magnet rotor with rotor poles ofalternating polarity around its circumference, the steps of:assemblingtwo stator phase assemblies each including a pair of pole pieces ofannular configuration in opposed relationship with each other to form anannular space therebetween, each said pole piece including along itsinner periphery spaced-apart salient stator poles, with the stator polesof the pole pieces of each stator phase assembly being interleaved withone another; orienting the stator phase assemblies such that the statorpoles on the pole pieces of one of the assemblies have a 90 electricaldegree index spacing from the stator poles on the pole pieces of theother assembly; permanently affixing a first pole piece of one of saidstator phase assemblies to a second pole piece of the other stator phaseassembly with the stator poles oriented to provide said index spacing,each of said first and second pole pieces having a plurality of spacedtabs around its periphery, the orientation of the stator phaseassemblies positioning the tabs on said first pole piece in overlappingfacing contact with the tabs on said second pole piece; forming a flatblank of magnetic material with side edges each including a plurality ofgrooves and protrusions and with equally spaced longitudinal groovesintermediate said side edges; bending said flat blank around the affixedfirst and second pole pieces with the overlapping tabs extending intosaid longitudinal grooves to form a field ring for said stator phaseassemblies, the field ring comprising a portion of a multi-part housingfor said machine; and positioning a pair of energizing windings inrespective cooperating relationship with said stator phase assemblies toenable the formation of magnetic fields in different phase relationshipwith each other.
 2. In a method as set forth in claim 1, the furtherstep of:positioning additional parts of said multi-part housing in amagnetic flux relationship with the side edges of said field ring withthe protrusions on said side edges disposed in grooves in saidadditional parts.
 3. In a method of making an electric rotating machinehaving a rotor with rotor poles of alternating polarity around itscircumference, the steps of:assembling two stator phase assemblies eachincluding a pair of pole pieces in opposed relationship with each other,each said pole piece including spaced-apart salient stator poles, withthe stator poles of the pole pieces of each stator phase assembly beinginterleaved with one another; orienting the stator phase assemblies suchthat the stator poles on the pole pieces of one of the assemblies have a90 electrical degree index spacing from the stator poles on the polepieces of the other assembly; permanently affixing a first pole piece ofone of said stator phase assemblies to a second pole piece of the otherstator phase assembly with the stator poles oriented to provide saidindex spacing, each of said first and second pole pieces having aplurality of spaced tabs around its periphery, the orientation of thestator phase assemblies positioning the tabs on said first pole piece inoverlapping facing contact with the tabs on said second pole piece;forming a flat blank of magnetic material with side edges and withequally spaced longitudinal grooves intermediate said side edges;bending said flat bank around the affixed first and second pole pieceswith the overlapping tabs extending into said longitudinal grooves toform a field ring for said stator phase assemblies, the field ringcomprising a portion of a multi-part housing for said machine; andpositioning a pair of energizing windings in respective cooperatingrelationship with said stator phase assembles to enable the formation ofmagnetic fields in different phase relationship with each other.
 4. In amethod of making an electric rotating machine having a rotor with rotorpoles of alternating polarity around its circumference, the stepsof:assembling two stator phase assemblies each including a pair of polepieces in opposed relationship with each other, each said pole pieceincluding spaced-apart salient stator poles, with the stator poles ofthe pole pieces of each stator phase assembly being interleaved with oneanother; orienting the stator phase assemblies such that the statorpoles on the pole pieces of one of the assemblies have a 90 electricaldegree index spacing from the stator poles on the pole pieces of theother assembly; permanently affixing a first pole piece of one of saidstator phase assemblies to a second pole piece of the other stator phaseassembly with the stator poles oriented to provide said index spacing,each of said first and second pole pieces having a flat annular baseportion and a plurality of spaced tabs extending around the periphery,the orientation of the stator phase assemblies positioning the tabs onsaid first pole piece in overlapping facing contact with the tabs onsaid second pole piece; forming a flat blank of magnetic material withside edges each including a plurality of grooves and protrusions andwith equally spaced longitudinal grooves intermediate said side edges;bending said flat blank around the affixed first and second pole pieceswith the overlapping tabs extending into said longitudinal grooves toform a field ring for said stator phase assemblies, the field ringcomprising a portion of a multi-part housing for said machine; andpositioning a pair of energizing windings in respective cooperatingrelationship with said stator phase assembles to enable the formation ofmagnetic fields in different phase relationship with each other.